Richard S. Siegel
Bronx High School of Science New York 68, N. Y.
I
Demonstrating the Relative Polarity of Non-Conducting Liquids
The rough correlation between dielectric constant and the polarity (dipole moment) of a molecule makes it possible to use determinations of dielectric constants as an indication of the polarity of a molecule. Normally, methods used for the determination of dielectric constants require complicated apparatus and an understanding of rather advanced principles. It thus becomes impossible to demonstrate differences in polarity of bonding to a high school or college freshman chemistry class. The apparatus to be describeiis based on the principle that an increase in the dielectric constant of the medium causes a decrease in the force of attraction between charged particles. The "charged particles" consist of two plastic heads about 1 cm in diameter. A bead is threaded on one end of each of two pieces of #I8 hare copper wire about 32 cm long. (If the fit is loose, the wire may be doubled or plastic cement may be used to give a tight joint.) A piece of aluminum foil ahout 4 cm square is wrapped tightly around each bead so that electrical contact is made with the wire. An open hook is formed a t the non-bead end of each of the wires by bending them about 2 cm from the end. These wire and bead electrodes are then suspended from stirrups made of heavy bare copper wire supported in the hole of a #5 or larger one-hole rubber stopper held by a buret clamp. The electrodes of a Wimshurst machine are then connected to the stirrups making sure that the leads are not grounded. A shallow glass dish is filled with the liquid to be tested to sufficient depth so that the beads can be submerged without touching the bottom of the dish. By adjusting the ringstands
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and buret clamps, the beads are lowered into the liquid. The electrostatic machine is cranked and attraction or lack of it is noted. The distance between the heads is then varied so that the maximum distance a t which attraction occurs may he noted. An alternate method of measurement is to keep the distance between the beads constant (3 or 4 mm) and note the relative amount of cranking of the electrostatic machine necessary to cause attraction. The author has used car* bon tetrachloride, chloroform, acetone, and water, the maximum distance of attraction varying from about 7.5 em for carbon tetrachloride to 0 for water. It is suggested that volatile, inflammable liquids of very low dielectric constant such as benzol not be used because of the danger of sparking outside the liquid causing a fire. While the writer has not experienced fires mith either chloroform or acetone, a fire extinguisher should be available during the demonstration. This apparatus can also be used in explaining the role of the solvent in the electrolytic dissociation process and the variation of the capacity of a condenser with the dielectric constant of the insulating medium.
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